1) Field of the Invention
The invention relates to ceramic evaporators.
2) Background Art
The most widely used method of coating flexible substrates with metals, in particular with aluminum, is high-vacuum web coating. The substrate to be coated is passed over a cooled roller and at the same time exposed to aluminum vapor which deposits as a thin metal layer on the substrate surface.
To produce the constant vapor stream required, ceramic evaporators known as evaporation boats, are heated to about 1450.degree. C. by direct passage of electric current. Aluminum wire is continuously fed in, liquefied on the ceramic surface and vaporized at a vacuum of about 10.sup.-4 mbar. In metallization units, a series of evaporation boats are arranged in such a way that a uniformly thick aluminum layer is deposited over the whole width of the substrate.
The chemical composition of the evaporation boats is generally 45-55% by weight of TiB.sub.2, 25-55% by weight of BN and 0-20% by weight of AlN. Such compositions are described in various patents (e.g. U.S. Pat. No. 3,915,900 or the patents cited in U.S. Pat. No. 4,089,643, column 1, lines 10 and 11, and column 2, line 5).
The customary shape of evaporation boats is based on a rectangular cross section. The widths and lengths of the evaporation boats are selected in accordance with the required evaporation rate of from about 0.35 to 0.40 g of Al cm.sup.-2 min.sup.-1. The rectangular cross section has proved useful because it makes possible a high mechanical stability, even at high temperatures, and inexpensive manufacture.
Evaporators having a non-rectangular cross section have also been described. U.S. Pat. No. 4,089,643 describes evaporators having a triangular cross section and a cavity. Significant advantages of the triangular evaporators over an evaporator having a rectangular cross section are the saving of material and the reduced electric power for operating such an evaporation boat. These advantages are offset by the serious disadvantage of a shorter operating life of the triangular evaporators compared to rectangular evaporators.